In amorphous silicon TFT (thin film transistor) forming processes included in an LCD (liquid crystal display) fabrication, an etching process must be carried out for plural times to achieve a complicated etching pattern associated with the TFT. Thus, with the conventional art, a photolithography process including an exposing process and a developing process is carried out for plural times corresponding to the number of the etching processes. Since different coating-and-developing apparatuses and exposure apparatuses are required for forming different patterns as etch masks, the total system cost is expensive.
As one solution of the above problem, a reflow process, which dissolves and deforms a resist film of a first pattern once used as an etch mask to reshape the same into a new, second pattern, attracts attention in these days. With the use of the reflow process, it is not necessary for forming the second resist pattern to perform processes employing a coating-and-developing apparatus and an exposure apparatus. The reflow process not only reduces the total system cost but also improves the production efficiency.
A series of processes for formation of an amorphous silicon TFT including a reflow process will be described with reference to FIG. 10. As shown in FIG. 10(a), on a gate electrode 201 formed on a glass substrate 200, an insulating layer 202, an Si-layer 203 composed of an a-Si layer (i.e., non-doped amorphous Si layer) 203a and an n+a-Si layer (i.e., phosphor-doped amorphous Si layer) 203b, and a metal layer 205 for forming drain and source electrodes are stacked in that order.
Then, in order to perform a first etching process that etches the metal layer 205, a resist film 206 having a first pattern is formed on the metal layer by a photolithography process including a photoresist film forming step, an exposing step and a developing step. A half exposing technique is used in the exposing step so as to obtain the resist film 206 having thickness distribution (i.e., the resist film 206 has thick portions and thin portions.). The half exposing technique employs a half-tone mask having light-transmittance distribution. The half exposing technique is described in US2004126713A1 (JP2005-108904A), for example. The resist film 206 having the first pattern is used as a mask for etching the metal layer 205, and portions of the metal layer 205 which are not covered with the mask are etched and removed, as shown in FIG. 10(b).
Altered layer 207 is formed in the surface region the resist film 206 due to application of a wet etching liquid used for etching the metal layer 205. The altered layer 207 is removed by supplying thereto an alkaline solution, prior to a reflow process, as shown in FIG. 10(c).
Then, as shown in FIG. 10(d), portions of the resist film 206 which are not necessary for a second etching process (i.e., the thin portions of the resist film 206) are removed by re-developing process, while portions of the resist film 206 near targets Tg (i.e., the thick portions of the resist film 206) remain.
Then, the remaining resist film 206 as shown in FIG. 10(d) is exposed to a solvent vapor-containing atmosphere. Thereby, the resist film 206 dissolves and diffuses (i.e., reflow) to move onto the targets Tg to cover the same. Thus, the resist film 206 is reshaped into a second pattern, in other words, a second resist pattern is formed. Then, the Si layer 203 is etched by using the metal layer 205 and the resist film 206 as masks, as shown in FIG. 11(a); and the resist film 206 is removed, as shown in FIG. 11(b). Thereafter, the n+a-Si layer 203 in channel regions is etched so that a TFT structure is formed, as shown in FIG. 11(d).
In the reflow process, the substrate contained in a processing chamber is exposed to a solvent vapor-containing atmosphere such a thinner gas-containing atmosphere, and the solvent that penetrates into the resist film dissolves the resist film. When supplying or discharging the solvent vapor-containing atmosphere into and from the processing chamber, the processing chamber is evacuated to reduce the internal pressure of the processing chamber from the standard pressure to a predetermined target pressure in order to improve the atmosphere replacement efficiency.
With the conventional art, the target pressure when introducing the solvent vapor-containing atmosphere into the processing chamber is the same as that when discharging the solvent vapor-containing atmosphere from the processing chamber; and the evacuating rate, or the pressure reducing rate, when introducing the solvent atmosphere is the same as that when discharging the solvent atmosphere. To be specific, as shown in FIG. 12, the pressure reducing rate is set to be high (e.g., 100 L/min in discharge rate), and the target pressure is set to be low (e.g., −90 kPa) to improve the atmosphere replacement efficiency. Note that, in this specification, the value of the “target pressure” is expressed as a differential pressure which is the target pressure minus the standard pressure. The standard pressure is typically atmospheric pressure.
In the second evacuating operation for discharging the atmosphere in the processing chamber, it is possible that the photoresist pattern may deform into an undesirable shape due to rapid evaporation of the solvent having penetrated in the resist film caused by rapid pressure reduction in the processing chamber. If the resist pattern is deformed, defects such as disconnections or short-circuits may be developed in the circuit pattern.